Processing phase change material to improve programming speed

ABSTRACT

A phase change material may be processed to reduce its microcrystalline grain size and may also be processed to increase the crystallization or set programming speed of the material. For example, material doped with nitrogen to reduce grain size may be doped with titanium to reduce crystallization time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/633,873, filed on Aug. 4, 2003 now U.S. Pat. No. 7,893,419.

BACKGROUND

This invention relates generally to the formation of phase changematerial memories.

Phase change memories use phase change materials, i.e., materials thatmay be electrically switched between a generally amorphous and agenerally crystalline state, as an electronic memory. One type of memoryelement utilizes a phase change material that may be, in oneapplication, electrically switched between generally amorphous andgenerally crystalline local orders or between detectable states of localorder across the entire spectrum between completely amorphous andcompletely crystalline states.

Typical materials suitable for such an application include variouschalcogenide elements. The state of the phase change materials is alsonon-volatile. When the memory is set in either a crystalline,semi-crystalline, amorphous, or semi-amorphous state representing aresistance value, that value is retained until reprogrammed, even ifpower is removed. This is because the program value represents a phaseor physical state of the material (e.g., crystalline or amorphous).

Thus, there is a need for phase change materials with desirable grainsizes and faster programming speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional view of one embodiment of thepresent invention;

FIG. 2 is an enlarged cross-sectional view of the embodiment shown inFIG. 1 at a subsequent stage of manufacture in accordance with oneembodiment of the present invention;

FIG. 3 is an enlarged cross-sectional view at a subsequent stage to FIG.1 in accordance with still another embodiment of the present invention;

FIG. 4 is a graph of voltage versus time for one embodiment of thepresent invention; and

FIG. 5 is a depiction of a system in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, a semiconductor substrate 12 may include a firstsubstrate portion 10 and at least a second substrate portion 14. In someembodiments, the substrate portion 10 may be P minus conductivity typematerial and the substrate portion 14 may be N conductivity typematerial.

A conductive heater 22 may be defined within an insulator 20 formed overthe substrate 12. The heater 22 may, in one embodiment, be formed oftungsten. The heater 22 is for applying heat to a localized region of anoverlying chalcogenide or phase change material 24. In one embodiment ofthe present invention, the material 24 is Ge₂Sb₂Te₅, also known as GST.

The phase change material 24 may be doped by adding small amounts ofnitrogen or nitrogen and oxygen into the phase change material tocontrol the number and size of the microcrystalline grains producedduring crystallization of the material. Nitrogen or nitrogen/oxygenco-doping at concentration levels of about 3 percent to about 6 atomicweight percent may be effective in producing a more uniform distributionof grains having sizes of from approximately 5 nanometers toapproximately 10 nanometers of crystalline phase change material. Suchgrains may be called micrograins.

For example, in one embodiment, a GST material 24 may be deposited by DCmagnetron reactive sputtering from a target with a nominal chemicalcomposition of GST. The chemical composition of the starting material 24may be Ge:Sb:Te=23.0:22.7:44.3 atomic weight percent in one embodiment.The sputtering power may be approximately 100 watts in this example. Thebackground pressure may be less than 1.4×10⁻⁵ Pa and the argon gaspressure may be less than 0.67 Pa, respectively. The nitrogen contentsof the GST layer 24 may be in the range of 0 to 12 atomic percent byvarying the nitrogen gas flow rate in the range of 0 to 10 sccm, and theargon gas flow rate may be fixed at 100 sccm. These GST materialcharacteristics are given for informational purposes only and are notintended to in any way limit the scope of the claimed invention.Nitrogen doping of GST films is described, for example, in Hun Seo, etal. “Investigation of Crystallization Behavior of Sputter-DepositedNitrogen-Doped Amorphous Ge₂Sb₂Te₅ Thin Films,” Jpn. J. Appl. Phys. Vol.39 (2000) pp. 745-751 (February 2000).

The nitrogen doped micrograin GST exhibits a reduction in thecrystallization speed compared to undoped GST. A phase change memory mayrely on a reversible crystalline to amorphous phase change inchalcogenide alloy semiconductor as its basic data storage mechanism. Amemory bit is programmed into the amorphous (reset) state by theapplication of a fast (for example, 1 to 10 nanoseconds) current pulsehaving sufficient amplitude to melt the chalcogenide alloy semiconductorin the programmable volume adjacent to the heater 22. When the resetprogramming pulse is terminated, the melted volume of chalcogenide coolsrapidly enough to freeze into a vitreous (atomically disordered) statethat has electrical resistance.

To program the bit into the low resistance microcrystalline (set) state,a lower amplitude pulse is applied that is sufficient to heat theprogrammable volume to a temperature just below the melting point. Theset pulse may be of a sufficient duration to permit the amorphousmaterial in the program volume to crystallize. For standard GST phasechange material, this set programming time is approximately 50nanoseconds.

The reduction in crystallization speed, as a result of nitrogen dopingthe micrograin GST, may result in unfavorable tradeoffs betweenprogramming current and programming speed. Since programming to the setstate already involves the longest programming pulse in undoped GST,further increasing the length of the set pulse with nitrogen dopedmicrograin GST may result in greater energy required for the programmingpulse, even though the programming current is reduced. In batteryoperated portable electronic equipment, for example, programming energyis more important than programming current since programming energydirectly impacts useful battery life.

In one embodiment of the present invention, the doping I may include,for example, nitrogen or nitrogen and oxygen to reduce grain size. Thedoping I may also include co-doping or simultaneous doping with titaniumto reduce the set programming time. For example, about 5 atomic percenttitanium may be used in one embodiment.

The titanium doping of the material 24 may be accomplished by directincorporation of titanium into the starting chalcogenide alloy in oneembodiment. For example, a sputter target used for physical vapordeposition of the phase change material 24 may incorporate titanium.Alternatively, a thin layer of titanium may be sequentially sputteredbefore or after the chalcogenide layer has been deposited. In stillanother embodiment, ion implantation of titanium may be utilized.

After the material 24 has been appropriately doped to include nitrogenor oxygen, as well as titanium, a metallic interconnect 28 may beapplied to form the completed device 11 as shown in FIG. 2. Thechalcogenide or phase change material 24 may include a volume made up ofpolycrystalline chalcogenide and a programmable volume 26 arranged overthe heater 22.

Referring to FIG. 3, in accordance with another embodiment of thepresent invention, a titanium source layer 30 may be formed over thephase change material layer 24 a, which has been doped only with thenitrogen or nitrogen/oxygen co-doping. The metallic interconnect layer28 may then be formed to result in the structure 11 a. Upon heating, forexample during semiconductor processing, titanium from the layer 30diffuses into the phase change material 24 a. In one embodiment, a 25Angstrom titanium film may be used with an 800 Angstrom GST film ofphase change material.

Referring to FIG. 4, a set pulse of approximately 10 nanosecondsduration is sufficient to program a test bit fabricated with titaniumdoped GST in one embodiment. Undoped GST devices of the same dimensionsuse a set pulse duration of approximately 50 nanoseconds or greater.

As shown in FIG. 4, the set time may be reduced. Phase change memorieswith approximately 400 Angstrom GST layers deposited on a 10 to 20Angstrom titanium layer may exhibit increases of set speed of greaterthan five times in one embodiment.

Turning to FIG. 5, a portion of the system 500 in accordance with anembodiment of the present invention is described. The system 500 may beused in wireless devices such as, for example, a personal digitalassistant (PDA), a laptop or portable computer with wireless capability,a web tablet, a wireless telephone, a pager, an instant messagingdevice, a digital music player, a digital camera, or other devices thatmay be adapted to transmit and/or receive information wirelessly. Thesystem 500 may be used in any of the following systems: a wireless localarea network (WLAN) system, a wireless personal area network (WPAN)system, or a cellular network, although the scope of the presentinvention is not limited in this respect.

The system 500 may include a controller 510, an input/output (I/o)device 520 (e.g., a keypad display), a memory 530, and a wirelessinterface 540 coupled to each other via a bus 550. It should be notedthat the scope of the present invention is not limited to embodimentshaving any or all of these components.

The controller 510 may comprises, for example, one or moremicroprocessors, digital signal processors, microcontrollers, or thelike. The memory 530 may be used to store messages transmitted to or bythe system. The memory 530 may also be optionally used to storeinstructions that are executed by the controller 510. During theoperation of the system 500 it may be used to store user data. Thememory 530 may be provided by one or more different types of memory. Forexample, a memory 530 may comprise a volatile memory (any type of randomaccess memory), a non-volatile memory such as a flash memory, and/orphase change memory that includes a memory such as, for example, memoryelements 11 or 11 a, illustrated in FIGS. 2 and 3.

The I/o device 520 may be utilized to generate a message. The system 500may use the wireless interface 540 to transmit and receive messages toand from a wireless communication network with a wireless radiofrequency (RF) signal. Examples of the wireless interface 540 mayinclude an antenna or a wireless transceiver, such as a dipole antenna,although the scope of the present invention is not limited in thisrespect.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A system comprising: a processor-based device; a wireless interfacecoupled to said processor-based device; a semiconductor memory coupledto said device, said memory including the substrate, said memory furtherincluding a layer of chalcogenide material over said substrate, saidchalcogenide material including a species to reduce the grain size ofthe chalcogenide material and a species to increase the crystallizationspeed of said chalcogenide material; an insulator over said substrateand under said chalcogenide material; and a heater extending throughsaid insulator to said chalcogenide material to heat said chalcogenidematerial.
 2. The system of claim 1 wherein the species to reduce grainsize includes nitrogen.
 3. The system of claim 1 wherein the species toincrease crystallization speed includes titanium.